Polymer Matrix Composites (PMC) are manufactured by embedding strong fibers such as, glass, carbon, aramid or natural fibers in a polymer. The composite materials benefit from the reinforcement provided by the strong fibers and have tensile, bending and impact strength properties much higher than non-reinforced polymers. Such composites find use in infrastructure, automotive, construction, aircraft and military industries. The polymer used in the composite, also known as the matrix, may be thermoplastic or thermosetting. Thermoplastic polymers are capable of melting upon heating with no change in chemical structure, whereas, thermosetting polymers are capable of chemically reacting. This converts the original, usually liquid, polymer to a rigid solid polymer that can no longer melt upon heating. The latter thermosetting polymers are used as liquid molding resins, successful examples of which include, but are not limited to, the well known unsaturated polyester, vinyl ester and epoxy resins. These resins are usually injected as a liquid into a mold containing the appropriate reinforcing fiber and are then cured in the mold to a rigid solid by the action of heat and catalyst. The successful liquid molding resins must have a low initial viscosity, have a long shelf life at room temperature, be capable of chemically reacting to a solid polymer by heating and/or addition of catalysts, must be able to react without the formation of volatile by-products, must have strong adhesion to the fibers used as the reinforcing agent and must have good physical properties such as, high ultimate tensile strength, fatigue resistance, impact strength, bending moment and high softening temperature.
Liquid molding resins are usually prepared by first synthesizing a low molecular weight polymer having the functional groups required for the cross-linking reaction. To achieve the desired low viscosity the polymer may be dissolved in a reactive diluent. If the cross-linking reaction is of the free radical addition type, the required functional group on the polymer is ethylenic unsaturation and the reactive diluent is also an ethylenicly unsaturated compound such as, but not limited to, styrene, .alpha.-methyl styrene, divinyl benzene, methyl methacrylate, etc. The relative ratios of unsaturated groups on the polymer and the amount of the reactive diluent are important parameters that those knowledgeable in the field have learned to optimize.
Just prior to use, liquid molding resins are mixed with catalysts and accelerators that start and facilitate the cross-linking reaction. If the cross-linking reaction is of the addition type, such accelerators as cobalt naphtenate, aromatic tertiary amines, etc., and free radical initiators such as, but not limited to, methyl ethyl ketone peroxide, benzoyl peroxide, cumyl hydroperoxide , etc., are added. The choice of initiators and accelerators depends on the reactivity of the polymer and the temperature and the time desired for the cure reaction. The choice of accelerators and initiators are well documented in the literature and are well known by those experienced in this field.
Successful liquid moldings such as, but not limited to, unsaturated polyesters, vinyl esters and epoxy resins are all synthesized using raw materials derived ultimately from petroleum. These include, but are not limited to, among others, maleic anhydride, phthalic acid, isophthalic acid, aliphatic diols, bisphenol-A, acrylic and methacrylic acid, aliphatic and aromatic diamines, all of which are petroleum derivatives. Replacing some, or all, of these petroleum derived raw materials with renewable plant-based raw materials is attractive, both economically and socially, as such raw materials are cheaper and their use contributes to global sustainability by not depleting scarce resources.
The use of plant-based raw materials such as plant oils is further useful as such naturally occurring compounds are usually consumed readily by microorganisms. In fact, plant triglycerides are readily hydrolyzed in vivo by lipase secreting bacteria. This would make polymers derived from such raw materials easily biodegradable in natural media. This aspect of these polymers is an additional advantage over polymers derived solely from petroleum based raw materials, very few of which are degradable by naturally occurring bacteria.
It is also the intention of the present invention to introduce a high modulus resin system that is suitable for composite formation using man made fibers such as, glass, carbon and aramid fibers as well as natural fibers, including, but not limited to, animal fiber (e.g., wool, cashmere, hair, bird feathers, etc.), and plant or vegetable fiber (cotton, sisal, fibrous cellulose, hemp, hay, straw, flax, jute, pine needles, etc.). In this manner, it is intended to produce composites whose matrix as well as reinforcement are predominantly made from natural and renewable materials. These materials are inexpensive and should find use in high volume applications such as, but not limited to, particle board for furniture and construction, engineered lumber, reinforced components for automotive, MDF panels for construction, ceiling panels and sculpture.
The use of epoxidized triglycerides, especially epoxidized soybean oil, is well documented. This compound is available in many levels of epoxidation and for the purposes of this invention, several are suitable: Paraplex G-62 available from C. P. Hall Company ; Chicago, Ill.; Drapex 6.8 available from Witco Co.; Taft, La. (having on the average 4.2 epoxy groups per triglyceride); and Vikoflex 7170 from Elf Atochem. The current commercial use of epoxidized soybean oil is as plasticizer for polyvinyl chloride.
The use of acrylated epoxy oils in various resins has also been investigated. European Patent 81973 discloses the use of acrylated epoxidized triglycerides to synthesize photo-polymerizable coatings for glass. In the Japanese Patent 73-98883, acrylated epoxidized triglycerides is used to prepare ink vehicles that are capable of photocuring. In U.S. Pat. Nos. 4,025,477, and 3,931,075, acrylated epoxidized triglyceride is treated with isocyanates and 2-hydroxyethylacrylate to give electron beam-curable coatings for metals with a Sward hardness of 14. In Japanese patent 75-126706, acrylated epoxidized triglyceride is used for photocurable ink vehicles used for textile printing. In Japanese Patent 73-98885, acrylated epoxidized triglyceride is used with toluene di-isocyanate and 2- hydroxyethylacrylate to give a co-polymer that is capable of photocuring in textile printing applications. In French Patent 76-37 678, acrylated epoxidized triglyceride is used as a photocurable high flexibility coating for leather. In Japanese Patent 78-26116, acrylated epoxidized triglyceride is used as a photocurable ink vehicle that gives a faster cure and higher gloss. In Japanese Patent 77-137522, acrylated epoxidized triglyceride is used in conjunction with glycidyl acrylate-octylacrylate co-polymer and alumina filler to prepare a potting compound used for fluorescent light fixtures that eliminates transformer humming. In European Patent 90-203517, the use of acrylated epoxidized triglyceride as electron beam or UV curable thermosetting inks, coatings, and adhesives, is disclosed. W. Shi et al., in J. Photopolym. Sci. Technol., 5, 453, (1992), describe acrylated epoxidized triglyceride resin for high-gloss UV cured coatings. All of the above references are incorporated by reference in its entirety, for all purposes.
Investigation of the literature shows that the prior art allows acrylated epoxidized triglyceride resins to be used as surface coatings only. These are necessarily flexible, lightly cross-linked amorphous polymers, with little or no structural strength. These substances have been used in the prior art as varnishes, adhesives, protective coatings, ink vehicles, and high-gloss surface treatments, none of which requires any structural strength. As will be apparent below, in this disclosure, new chemical reactions and modifications allow epoxidized triglycerides to be polymerized to higher molecular weights and higher cross-link densities so that the new resins have structural strengths comparable to those of other liquid molding resins now in commercial use. Such use includes, but is not limited to, the high volume composite utilization fields of civil infrastructure, defense, aerospace, marine offshore, construction, bridge rehabilitation, automotive, farming equipment, electronics, etc.
Ring opening cure reactions of epoxy resins are well known. The use of diamines, anhydrides, dicarboxylic acids and diols have been reported in the literature. These reactions are exceptionally easy when the epoxy ring that is undergoing the reaction is terminal, that is, at the end of the molecule, which is the case in all commercially successful epoxy resins. In epoxidized triglycerides, however the epoxy group is necessarily internal, such that it is substituted on both sides by bulky alkyl groups, rendering it far less reactive towards the traditional ring opening polymerization reactants. Some of the resins described in this disclosure use the epoxy functional group of these triglycerides with various diols, diamines, anhydrides and diacids to produce highly cross-linked network polymers by ring-opening polymerization reactions.
The advantage of ring opening polymerization reactions is twofold: First, there is no by-product during the ring-opening polymerization, as all reactions are of the addition type; second, the degree of crosslinking, and therefore the final properties of the cured resin, can be controlled by merely adjusting the stoichiometry of the epoxy component and the second reactant, e.g., diols, anhydrides, diamines, dicarboxylic acids, alkoxides, etc.
Another advantage of such thermally induced epoxide ring-opening polymerization is, that unlike free radical addition reactions, ring-opening reactions can be stopped and restarted at will by decreasing or increasing the temperature, respectively. This allows individuals knowledgeable in the art to synthesize pre-polymers of desired molecular weight and viscosity, which can be kept at room temperature indefinitely (A-Stage), but which can be cured to a solid state in a mold merely by the application of heat (B-Stage).
Commercially successful epoxy resins usually consist of a two component system, the epoxy prepolymer and the curing agent, both as separate components. These components have to be metered, weighed, and mixed by the end user. The avoidance of two-component cure systems makes the resins described herein more attractive, as the end user need not be concerned with these complicated and error-prone mixing and metering steps.
Another advantage of the resins described in this invention is that the physical state and rigidity of the product can further be manipulated by the addition of various co-reactants having reduced or increased reactivity so that a desired fraction--or, in fact, all--of the epoxy groups are used in the final stage of the reaction. Such co-reactants include, but are not limited to, primary and secondary alcohols and primary and secondary amines. The rate of the ring-opening polymerization can be adjusted by using desired amounts of ring-opening catalysts, which include, but are not limited to, cyanoguanidines, imidazoles, Lewis acid, metal alkoxides, and bases.
An added novelty of this disclosure is that the epoxy content of the epoxidized oils depends on the level of unsaturation of the oil used as raw materials. It is well known that oils from different plants such as, but not limited to, cotton, sunflower, corn, soy bean, and linseed have different amounts of unsaturation. Depending on the property desired in the final product, various oils, or mixtures thereof, may be used for the epoxidation reaction. Therefore, a raw material of the exactly desired epoxy equivalent can always be obtained by mixing epoxidized triglycerides from different plants. In this disclosure, the variation of unsaturation among the various plant oils is used to advantage. A promising development in this field is the future availability of triglycerides from genetically engineered plants which contain much higher levels of unsaturation and controlled distribution of fatty acid chain length than currently attainable from the natural plants.
The resins disclosed herein are more affordable than the hitherto commercially available liquid molding resins; their manufacture involves simple reactions that require simple reactors and machinery, and their origin from renewable resources makes them environmentally friendly and supportive of global sustainability.
Ring opening polymerization of epoxidized triglycerides has been disclosed in U.S. Pat. No. 3,291,764 where triethylenetetramine, p-phenylenediamine, phenylbiguanidine, etc., have been used as curing agents to give semi-fluid resins that were soluble in aqueous acid and used as a surface coating. Japanese Patent 73-102647 describes mixtures of epoxidized triglycerides, commercial epoxy resins such as Epikote 828 and diaminodiphenylmethane to give moldable solids; Frishinger, in Adv. Che. Ser. 239, 539, (1994) describes mixtures containing small amounts of plant triglycerides and mostly commercial epoxy resins and epoxy curing agents for the purposes of toughening commercial epoxy resins.
Roesch, et al., in Polymer Bull. (Berlin) 31,679, (1993) describes reaction of epoxidized triglycerides with maleic, succinic, hexahydrophtallic norbornanedicarboxylic and phthalic anhydride. The reaction is run in a polypropylene melt with small amounts of epoxidized triglyceride dispersed in the melt where the dispersed phase is the epoxidized triglyceride and the continuous phase is polypropylene. The aim of this work is to produce toughened polypropylene.
Hydrogen peroxide oxidation of triglycerides to epoxidized triglyceride has been described by R. Oda, in Journal Society of Chemical Industry Japan, 41, 195-195 (1938) and by Y. Isii, in Journal Society of Chemical Industry, Japan, 43, 255-7, 315-7, 374-9 (1940)., and by Swern and Billen in Journal of Organic Chemistry, 67, 1786, (1945).
Maleinization of triglycerides have been the subject of many publications: Teeter, in J. Org. Chem. 22, 512, (1957) describes the reaction of maleic anhydride with conjugated fatty acids; Bickford, in J Am. Oil Chemist's Soc., 25, 254, (1948) describes maleinization of unconjugated triglycerides. Plimmer, in J. Oil Color Chemists' Assoc., 32, 99 (1949) describes the reaction of a number of different triglycerides with maleic anhydride. Maleic anhydride is known to react with triglycerides in an ene reaction, as well as insertion reactions giving oligomeric triglycerides. The procedures described in these papers are used to prepare maleic modified oils used in varnish manufacture. The reactivity of the anhydride has been used to react it with 2-hydroxyethylmethacrylate to give free radical curable resins for surface coating applications, as described in the Japanese Patent 81-64464, and for UV curable coatings, as described in the German Patent 89-3938149.
Glycerolysis of triglycerides has been known since antiquity. This is the traditional starting material for alkyd resins used as binders for "oil paints." There are many references to the formation of monoglyceride oils. There are also numerous references to polyesterification of monoglycerides with various diacids and dianhydrides. The comonomers that have been used are phthalic anhydride, fumaric acid, pentaerythritol, glycerol, and aliphatic diacids, such as, but not limited to, succinic, glutaric, and suberic acid. The common point among the previous work is the formation of polyesters with saturated diacids and the use of the unsaturation contained in the fatty acid itself for "air drying", that is, peroxidative cross-linking of the products.
Monoglyceride polyesters have been described in the literature: For example, Japanese Patent 74-103144, describes phthalic anhydride alkyd for air drying paints; U.S. Pat. No. 3,827,993 describes diethylene glycol-phthalic anhydride alkyd for surface coating; U.S. Pat. No. 4,740,367 describes fumaric acid alkyd used as an emollient base for skin and hair care products; Japanese Patent 73-125724 describes phthalic anhydride and pentaerythritol alkyd used for acid curable coatings; Japanese Patent 74-91317 describes phthalic anhydride and glycerol alkyd used for storage stable coatings; Japanese Patent 78-52321 describes phthalic anhydride and pentaerythritol alkyd used for air curable, water resistant coatings; Japanese Patent 80-62752 describes phthalic anhydride alkyd used for tough, air drying, glossy coatings; Japanese Patent 84-254873 describes isophthalic acid and polyethylene glycol alkyd for surface coatings.
Amidation of triglycerides has been discussed in a number of sources. Fatty acid amides are commercially important substances used as antistatic and softening agents for textiles. In British Patent 1248919, fatty acid amides made from fatty acids and diethanolamine to produce foamed resins are disclosed. There is no work in the literature on direct amidation of triglycerides, maleinization of the diethanolamide with maleic anhydride or on the polymerization of the maleate half-esters. The resin system disclosed here does not involve a polyester. It is only a half-ester of maleic anhydride--that is, a 1:2 adduct of the monoglyceride with maleic anhydride. This product is formed without any by-product by the reaction of the plant monoglyceride hydroxyl groups with maleic anhydride. The resulting molecule is unique and has not been synthesized before. The resin system disclosed herein does not depend on the air oxidation of the fatty acid unsaturation; in fact, those double bonds are intact in the product. The resin system described in this invention cures via the co-polymerization of reactive diluent and the maleate half-esters. In this invention, maleic anhydride is used as the esterification reagent and, among other reactions and processes, excess glycerol is used as a means of adjusting the cross-link density, thereby producing a structurally strong thermoset from natural triglycerides. The new thermoset and its modifications is recommended for use in composite manufacturing with high-modulus synthetic and natural fibers.